A Solar-Assisted Uninterruptable Power Supply
What is a solar-assisted UPS?
Like a regular UPS it has a battery, inverter, charge controller
and transfer switch. Unlike a regular UPS there is also a solar
panel to charge the battery. The control circuitry is different
and the battery larger, because the battery is only charged
partially by the AC power supply, leaving enough capacity for
daily solar charging.
Why might you want to build one?
Because you'd like a low-budget solar system that doesn't involve
any red-tape, letting you offset retail electricity with all the
solar power you can generate, and still use AC at night or on
Because you'd like an alternative to a generator when the power
goes out for extended periods (e.g. to keep a freezer running).
Because you want to learn about solar power and would enjoy live
plots on your website of solar power generated and used.
Because you'd like to invest in something more dependable than the
Because solar-assisted UPSs are unfortunately not yet commercially
Why might you not want to build one?
Because the sulfuric acid in the battery is corrosive, the hydrogen
gas it produces is explosive, and the lead is a neurotoxin.
Because wiring together a 12V system (like the ones found in cars)
requires skill, mistakes could damage equipment or cause injury.
Because you don't have a computer or need a UPS.
What parts are needed? How much might it cost?
- conventional UPS - hopefully surplus with a dead battery.
- 17V solar panel(s) - big enough to provide between half and
100% of your intended load ($330 for 80W)
- 12V deep cycle lead-acid battery, twice as large as the
maximum daily output of the solar panels ($110 for 100AH)
- control system - including a computer, digital voltmeter
and computer-controlled AC outlet ($112 without PC)
- panel rack, fuses and cabling - about $100
How to Build a solar-assisted UPS
The above task list is designed to prevent disasters in the form of
equipment purchased that doesn't ever become productive. The PV panel
is the most expensive item, and its purchase is put off until the rest
of the system is functional. Also, the computer-related things are
bought first, so you know right away if they will work for you. The
battery needs to be purchased after your charging system is in place,
because batteries self-discharge and can be damaged if kept at a low
state of charge for months. The suggestion that you half-discharge your
battery is intended for true deep cycle batteries, which will not be
damaged by such a deep discharge. With computer control, you could even
use a bank of starter batteries, if you cycled them very shallowly.
- Order automation/monitoring hardware and get it working on your PC.
I bought my Phidgets voltmeter from Trossen Robotics
and used an x10
appliance module for the outlet. Since I wanted full details on
the productivity of my system, I also got 2 Phidgets ammeters.
- Purchase a fuse block and fuses from an automotive parts supplier or
from Del City .
Get 16 gauge wire by cutting up an outdoor extension cord. For
the solar panel wiring, you can use an extra heavy outdoor
extension cord with 12 gauge wire.
- Obtain a conventional UPS (ask your computer people at work or
open up its battery compartment, remove the old battery, and wire
the cabling there to the fuse block. I connected the UPSs crimp
connectors to my own crimp connecters with two small tabs of
- Purchase a new and much larger battery. Make sure the UPS can
charge and use the battery without any solar connections yet.
When shopping for your battery be careful to purchase a fresh
one according to the manufacturing codes explained here
If you are not using a solar charge controller, the battery must
be flooded, not AGM or sealed.
- Wire your digital voltmeter to the fuse block, and plug the UPS
into a computer controlled outlet. Write a control script to
regulate the battery voltage, only powering the UPS when the
battery is less than half charged (less than 11.9V).
- Order a PV panel and optionally a solar charge controller. Wire
these into the fuse block, and watch the voltage climb as the
sun charges your battery. Install ammeters if desired to monitor
how much current is produced and consumed by your system. If
you have no solar charge controller, then be sure to use enough
power to keep the battery below 14V most days. The battery does
need to be equalized monthly, at higher voltages, but very high
voltages could burn out the inverter in the UPS. You can manually
move your load to utility power, turn off and disconnect the UPS
and let the battery equalize on a sunny day when you are home and
can stop it before it gets above 15.5V.
- Build a rack for the panel using ordinary construction materials.
I cut apart a 10 foot antenna mast and bolted it to the long sides of the panel,
letting the ends stick out several inches. I put the ends into
small cinderblocks that hold them tightly, and placed 2 of the small
blocks on top of larger ones to give the panel the proper tilt and
face it towards the equator. I built a reflector from foil-faced
plastic insulation, wired to hardware cloth which is in turn wired
to additional cinder blocks. This rack is low cost, adjustable,
stable in high winds, and doesn't require a permanent structure.
About the Economics
I spent about $750 building my 80W solar-assisted UPS, not counting the
conventional UPS and the controlling computer. Normally a UPS is
intended to power a computer, so I am presuming that you have one of
these already. If you are buying a computer only to control the UPS,
the economics and productivity of the system are drastically reduced,
and I'd recommend against it. A conventional UPS costs only about $100,
so you could purchase one of those without blowing the budget. My
solar-assisted UPS powers a laptop computer, which is also serving as a
webserver, mailserver, television, security system and wireless access
point. Today's machines are so powerful that the machine in question is
very lightly loaded and draws only 18W. If the primary goal of your
project is to save electricity, and you are running a desktop server I'd
urge you to replace it with a laptop, because the electricity you save
by doing so is much more than you'll be able to produce with the modest
solar-assisted UPS described here, and the cost is less. My server cost
only $450 including shipping, and the solar panel on my UPS is providing
about 70% of the power it uses, or $15 worth of electricity per year.
That means my solar-assisted UPS is producing approximately a 2% yield,
tax free and increasing in value as electricity rates increase. I spent
a little extra money on additional monitoring of my system. A basic
solar-assisted UPS could have a slightly higher yield, but a lot depends
on how much sunshine you have in your climate, and how much you spend on
When commercially produced solar-assisted UPSs become available their
economics should be more favorable, because they should contain their
own control circuitry, and you could use them without a computer. Since
the yield on investment for a home built solar-assisted UPS isn't that
different than the yield on a savings bond (1.3% for series E November
2008) it could be argued that small scale solar power is close to grid
parity. In fairness, I should admit that a solar-assisted UPS requires a
lot more spare parts and effort on the part of the owner than a savings
bond. True grid parity isn't quite here yet. The economics of a
solar-assisted UPS may be more favorable than the economics of a grid
inter-tie solar system, (apart from tax credits) because the engineering
documentation requirements are far more extensive for a
power plant than for an appliance. A solar-assisted UPS doesn't require
a building permit, because no permanent alterations are made to the
building, and the
connection to building wiring is through an ordinary plug. This makes it
practical to install one if you're renting or living with family.